Metal containers such as beverage can or battery container are formed by drawing, drawing and ironing, drawing and stretch forming, or ironing after drawing and stretch forming with the object of material reduction and extension of inside measurement by reduction of wall thickness of container. These metal containers are generally coated inside to give corrosion resistance for content and printed after being coated outside to show content. However, a metal sheet previously covered with organic resin is tried to apply to above-mentioned heavily formed use with a view to reduce coating cost and to eliminate environmental pollution caused by dispersing of solvent during coating operation. And cans formed of metal sheet covered with organic resin has already been placed on sale in beverage can market.
In the organic resin covered metal sheet applied to the above-mentioned heavily formed use, a biaxially oriented film, which is manufactured by biaxial elongation of a thermoplastic polyester resin and subsequent heat-setting of it, is heat bonded to a metal sheet. The biaxial orientation of the film before heat bonding is partially or entirely lost from the contacting portion of the film to the metal sheet to the free surface (the surface not contacting to the metal sheet) in the thickness direction of the film by the heat conducted from the metal sheet when the film is heat bonded to the metal sheet. When the biaxial orientation of the film is totally lost after heat bonding, it causes the excellent adhesion between the film and the metal sheet for the above-mentioned heavily formed use, which is favorable for preventing peeling-off of the film and generation of film cracks during forming operation. On the other hand, the film without orientation has so large permeability that the content permeates the film and corrodes the metal substratum, and furthermore, it has faults that coarse spherlites are generated in the film by post heating during the operation of printing which shows the content or the like and cracks are easily caused in the film by falling of container or collision of each container.
Therefore, in the metal sheet covered with polyester resin film having biaxial orientation applied to the above-mentioned heavily formed use, the biaxial orientation of the film after the heat bonding is controlled to have formability compatible with permeation resistance and impact resistance (Laid open Japanese patent Hei 6-329669).
So far, copolyester resin obtained by copolymerization of ethylene terephthalate and ethylene isophthalate has been used in the organic resin covered metal sheet applied to the above-mentioned heavily formed use, and polyethylene terephthalate without copolymerized component has not been used in that field. The following is the reason. That is, copolyester resin obtained by copolymerization of ethylene terephthalate and ethylene isophthalate has small crystallization velocity, which undergoes little change of biaxial orientation by the temperature change of the metal sheet when a resin film is covered on a metal sheet by a heat bonding method consisting of contacting a resin film to a metal sheet heated to a temperature more than melting temperature of covering resin and pressing both, and it is rather easy to make biaxially oriented film structure having aforementioned formability compatible with permeation resistance and impact resistance by partially losing biaxial orientation before heat bonding as mentioned before. On the other hand, polyethylene terephthalate has large crystallization velocity, which causes extreme difficulty making biaxially oriented film structure having aforementioned formability compatible with permeation resistance and impact resistance by partially losing biaxial orientation before heat bonding when the film is covered on a metal sheet by heat bonding method. That is, as the biaxial orientation largely changes by a little temperature change of the metal sheet, it is extremely hard to make a prescribed film structure after covering. However, as copolyester resin has small crystallization velocity, it has a fault that crystal is apt to grow into coarse spherlite when it is post heated in the aforementioned printing operation or the like, which causes larger extent of deterioration of impact resistance. Furthermore, the price of copolyester resin film is high. Therefore, a polyester resin film of which film structure is easily made into favorable state after heat bonding as that of copolyester resin film and of which price is low is required.